Inorganic nanocrystals have been used in hybrid organic-inorganic or all-inorganic solar cells [W. Huynh et al., Science, Vol. 295, pp. 2425-2427, 2002; I. Gur et al., Science, Vol. 310, pp. 462-465, 2005.] Power conversion efficiencies in the range of 2-3% have been demonstrated. In these devices, nanocrystals based on binary compound semiconductors are synthesized and processed in solution.
To efficiently absorb light in the broad solar spectrum (spanning from ultraviolet to visible to near infrared), the energy gap of the light absorber in the solar cells needs to be optimized. At the same time, the energy level alignment between different materials in the active region will have to be optimized as well to minimize the energy loss during charge generation and transport. This means that the band gap (the energy difference between the conduction and valence bands) and the electron affinity (the energy difference between the vacuum level and the conduction band) need to be independently optimized. Due to the quantum confinement effect, the band gap of a nanocrystal can be tuned to some extent by controlling the size of the nanocrystal during the synthesis process. The electron affinity will be varied accordingly. Therefore, it is unlikely that the use of binary compound semiconductors, which have fixed stoichiometry, will achieve the optimized band gap and the optimized electron affinity simultaneously.
Organic solar cells have the potential to provide very low cost solar energy conversion due to many technological advantages of organic semiconductors, such as low material cost, ease of processing, and compatibility with flexible substrates. There have been considerable interests in organic-based photovoltaic (OPV) cells in the last two decades, and the power conversion efficiency of OPV cells has steadily improved to the current record of approximately 5%. However, such efficiencies are still far lower than the theoretical limits, and it is imperative to greatly improve the efficiency to make this technology suitable for large-scale commercial applications.
In inorganic solar cells, the absorption of incident light leads to free positive and negative charge carriers in the semiconductor, which are then transported toward opposite electrodes under the influence of the built-in electric field. In organic semiconductors, because of the different fundamental properties from their inorganic counterparts, light absorption only leads to tightly bound charge pairs, called excitons, each having one positive charge (hole) and one negative charge (electron). A hetero-interface between two different organic semiconductors is generally used to split or dissociate the excitons and create free positive and negative charges, which are then transported toward opposite electrodes under the influence of built-in electric field, leading to a photocurrent and/or a photovoltage.
The excitons, which are neutral, if created away from the interface, may diffuse within the organic semiconductors through random hopping between neighboring molecules. However, typically the average distance these excitons can diffuse is only about 10 nm, nearly an order of magnitude shorter than the organic layer thickness (50 to 100 nm) needed for efficient absorption of the incident light.
One way to address the exciton-diffusion problem is to find materials that exhibit very long exciton diffusion lengths that are comparable with the necessary thickness (50 to 100 nm) for efficient absorption of incident light. While some organic materials can have slightly longer exciton diffusion lengths, it is generally difficult to find or synthesize such materials that are suitable for organic solar cells. One such example is fullerene (C60), which has a reported exciton diffusion length of 40 nm. The current state-of-the-art organic solar cells typically use C60 or one of its soluble derivatives as one of the two materials in the photoactive region. C60, however, is not a very strong absorber for light with wavelength longer than 530 nm.
Mixing two or more organic semiconductor materials to form a so-called bulk heterojunction can circumvent the exciton-diffusion problem associated with organic semiconductors, as the interface between the different organic semiconductors in the bulk heterojunction is just one or a few molecules away from every exciton generation site. Such a bulk heterojunction can be formed by various processes, e.g., co-evaporation of two vacuum-sublimable molecules in a vacuum deposition system, spin coating or ink-jet printing of a blended solution of two different organic materials. However, the collection of photogenerated charge carriers depends on the charge transport properties of the bulk heterojunction, which in turn is strongly affected by the morphological structure of the material mixture. As the transport of charge carriers toward the respective electrodes relies on hopping between the same species of molecules, the random mixing of two different materials generally leads to inferior charge transport properties, i.e., lower charge carrier mobilities. Thus, a significant portion of the charges may not be able to reach the respective electrodes and be collected, partially negating the overall efficiency gain due to the improved exciton diffusion/charge generation process.
Based on the spins of the constituent positive and negative charges, an exciton can have a total spin of either 0 or 1, corresponding to the singlet and triplet state, respectively. Due to spin conservation, the transition between the ground state and the singlet exciton state is allowed, whereas it is forbidden between the ground state and triplet state. Therefore, compared with a singlet exciton, a triplet exciton has a much longer lifetime, as in the case of C60. This means that compared with a singlet exciton, a triplet exciton tends to diffuse farther before recombination, or before it “dies”. Diffusion length as high as 150 nm for triplet excitons has been reported in 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP, a wide-gap material commonly used as the host for a luminescent dye in an OLED). However, triplet excitons have a much lower probability to be formed directly upon light absorption, again due to the disallowed transition from the ground state to the triplet exciton state. This means that the absorption corresponding to such triplet exciton states is generally much weaker than that to singlet excitons.
Accordingly, there is a need for an apparatus that can efficiently absorb light and allow the charged carriers created to diffuse to collection electrodes so as to be efficiently collected.